A convenient way of probing the integrity and measuring the attenuation of a subscriber line in a telephone system is to transmit a pilot tone preferably in the voice-frequency range, e.g. of 800 Hz, over the line and to detect the amplitude level of that pilot tone at a remote end of the line. Thus, for example, an operator may attach to the line a service telephone and dial a particular call number which connects a pilot-tone generator across the line at the associated central office; a level-measuring device such as a voltmeter, receiving the incoming pilot tone, then allows that operator to check whether a discontinuity or other fault unduly attenuates the signal. For proper testing, however, the following conditions must be satisfied:
(a) the line loop must remain intact for direct current, i.e. the line must be conductively terminated at the testing station by a resistance--e.g. of 150 ohms--corresponding to that normally represented by the subscriber's telephone set during a conversation; PA1 (b) the incoming pilot tone must see a terminating impedance equal to the characteristic line impedance, e.g. of 600 ohms; PA1 (c) the service telephone used for calling in the remote pilot-tone generator must be disconnected during the test without interrupting the line loop for more than a few milliseconds, in order to avoid the transmission of a disconnect signal cutting off that generator.
It should be noted that the term "service telephone", as used herein, could also encompass the subscriber's regular set if it is disconnectable from the line by a plug-and-jack coupling.
Conventional equipment designed to carry out these operations is often cumbersome and heavy; in many instances, its handling requires specially trained personnel.